Method for efficiently separating lignocellulose and realizing full-component utilization
Technical Field
The invention relates to a method for efficiently separating lignocellulose and realizing full component utilization, in particular to a method for pre-treating lignocellulose in a solvent, realizing separation of lignin and solid cellulose substances and catalytically converting the solid cellulose into ethylene glycol.
Background
Lignocellulose is the most important biomass resource and has the advantages of wide source and abundant reserves, so that the high-efficiency utilization of the lignocellulose attracts the general attention of people. However, hemicellulose, cellulose, lignin and a small amount of impurities in lignocellulose are combined together to form a compact protective layer, so that the lignocellulose is difficult to be catalytically converted and utilized. The lignin is removed by alkali in the conventional pretreatment, and acid is used for neutralization in the subsequent process, so that the aim of precipitating the lignin is fulfilled. The process consumes a large amount of acid and alkali and produces salt, and has the characteristic of being not environment-friendly.
On the other hand, small molecular alcohols such as ethylene glycol are important energy source liquid fuels and also are very important polyester synthetic raw materials, for example, used for polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), and also used as an antifreeze, a lubricant, a plasticizer, a surfactant, and the like, and are organic chemical raw materials having a wide range of applications.
The method for preparing the ethylene glycol by utilizing the renewable biomass can reduce the dependence of human on fossil energy substances, and is beneficial to realizing environment friendliness and economic sustainable development. Cellulose-based substances including cellulose, hemicellulose, crop straw, corn cob, rice straw, wheat straw, miscanthus, pine, birch, poplar, etcn(H2O)mA compound which is widely found in nature. With the development of agricultural technologies, the yield thereof is increasing. The carbohydrate is developed to prepare the micromolecular alcohols such as ethylene glycol, propylene glycol and the like, so that the dependence on petroleum resources can be reduced to a certain degree, and the deep processing of agricultural products to prepare high-added-value chemicals is facilitated.
Ethylene glycol is currently prepared by catalytic hydroconversion of cellulose under hydrothermal conditions (reference 1: Direct catalytic conversion of cellulose into cellulose ethylene glycol using a nickel-heated catalyst, Angew. chem. int. Ed.2008,47, 8510. 8513; reference 2: Transition metal-branched dimethyl catalyst for the conversion of cellulose ethylene glycol, ChemUSM 2010,3, 63-66; reference 3: CN 101735014A, a process for the preparation of carbohydrates; reference 4: CN 102190562A, a process for the preparation of ethylene glycol from carbohydrates). The method uses a mixed catalyst composed of a tungsten-based catalyst and a hydrogenation catalyst to perform catalytic conversion on cellulose, thereby obtaining 60-75% of ethylene glycol. Similarly, a two-component catalyst composed of tungsten in an oxidized state and a hydrogenation metal can realize the high-selectivity preparation of ethylene glycol and propylene glycol from sugar-containing compounds such as cellulose and starch under hydrothermal hydrogenation conditions (document 5: a method for preparing ethylene glycol from a polyhydroxy compound, WO 2011113281A).
In the processes, the selectivity of ethylene glycol is good, the yield is high, but the reaction raw material is cellulose, an acid-base process extracted from lignocellulose still exists, and the environmental friendliness of the whole process is poor. When straw is used as the raw material, the yield of ethylene glycol is significantly reduced, even after alkaline pretreatment, to less than 40% (document 6: Catalytic hydrogenation of corn residue to ethylene glycol and 1,2-propylene glycol. "Industrial & Engineering Research 50,11(2011): 6601-. Therefore, there is a need to develop a cheap and environment-friendly method for separating lignocellulose, which can realize the efficient separation of lignocellulose and convert the treated cellulose substances into glycol efficiently.
The method provided by the invention takes lignocellulose as a raw material, separates lignin and cellulose substances through solvent pretreatment, and catalytically converts the cellulose substances into ethylene glycol. The method is simple to operate and low in cost, realizes full component utilization of the lignocellulose, and obviously improves the catalytic conversion efficiency of cellulose substances in the lignocellulose and the space-time yield of the dihydric alcohol.
Disclosure of Invention
The invention aims to provide a method for efficiently separating lignocellulose and realizing full component utilization, the effective separation of lignin and cellulose is realized through pretreatment, the obtained cellulose can be efficiently converted into glycol, and compared with the conventional separation process, the method has the advantages of high lignin purity, low impurity content, high time-space yield of dihydric alcohol, few byproducts and easiness in industrial production.
In order to achieve the purpose, the invention adopts the technical scheme that:
taking natural lignocellulose as a raw material, taking one or more of formic acid, acetic acid, propionic acid, oxalic acid and maleic acid as a solvent, adding hydrogen peroxide or/and ozone, and pretreating the lignocellulose at a certain temperature; filtering after pretreatment to obtain filtrate and solid insoluble cellulose, wherein the solid insoluble cellulose is used for preparing polyol by catalytic conversion; distilling the filtrate to recover the solvent and obtain residual solid component lignin;
the solid cellulose material is catalytically converted in water in a closed high pressure reactor, the adopted catalyst is a composite catalyst and comprises a catalyst A and a catalyst B, the active component of the catalyst A is one or more than two of transition metals of groups 8, 9 and 10, such as iron, cobalt, nickel, ruthenium, rhodium, palladium, iridium and platinum, and the catalyst B is one or more than two of tungstic acid, ammonium metatungstate, tungsten oxide and tungsten bronze; stirring and reacting in a reaction kettle; hydrogen is filled in the reaction kettle before reaction, the reaction temperature is more than or equal to 120 ℃, and the reaction time is not less than 5 minutes.
The lignocellulose refers to biomass containing lignin, cellulose and hemicellulose, and comprises one or more of crop straw, corn cob, rice straw, wheat straw, miscanthus, palm skin, pine, birch and poplar.
The particle size of the crushed lignocellulose particles is less than 5cm, the water content is less than 20%, the water soluble substance content is less than 10%, namely the content of substances which can be dissolved in water at room temperature is less than 10%.
The mass ratio of the lignocellulose to the solvent is between 10:1 and 1: 10; the solvent is one or more of formic acid, acetic acid, propionic acid and oxalic acid; the mass of the hydrogen peroxide and/or the ozone accounts for 0.001 to 5 percent of the total lignocellulose; the pressure of the ozone pretreatment is 0.2-1MPa, the pretreatment temperature is 20-100 ℃, and the pretreatment time is 0.2-5 hours.
The pretreated filtrate contains lignin, a small amount of low molecular sugar and a solvent; the solvent is recycled by distillation, and the loss rate of the solvent is lower than 5%; the residual solid substance obtained by washing the residual solid after distillation with water and filtering is lignin, and the low molecular sugar dissolved in water can be used together with cellulose for preparing the polyalcohol by catalytic conversion.
The pretreated solid cellulose substance is a reaction substrate rich in cellulose, the cellulose content is more than 80%, and the solid cellulose substance contains a small amount of hemicellulose and lignin.
The solid cellulose substance is used for the reaction of preparing glycol by catalytic conversion, the reaction is carried out in water in a closed high-pressure reaction kettle, the adopted catalyst is a composite catalyst and comprises a catalyst A and a catalyst B, the active component of the catalyst A is one or more than two of transition metals of groups 8, 9 and 10, such as iron, cobalt, nickel, ruthenium, rhodium, palladium, iridium and platinum, and the carrier is one or more than two of active carbon, alumina, silicon oxide, silicon carbide, zirconium oxide, zinc oxide and titanium dioxide; the catalyst B is one or more than two of tungstic acid, ammonium metatungstate, tungsten oxide and tungsten bronze; stirring and reacting in a reaction kettle; hydrogen is filled in the reaction kettle before reaction, the reaction temperature is more than or equal to 120 ℃, and the reaction time is not less than 5 minutes.
The catalyst A is one or more of skeleton nickel, ruthenium/carbon, iridium/carbon, ruthenium/titanium dioxide, nickel-ruthenium/active carbon and nickel-iridium/active carbon; the nickel content of the framework nickel is more than 80 wt%, and the metal loading of the supported catalyst is 1-10 wt%; the catalyst B is one or more of tungstic acid, ammonium metatungstate and tungsten oxide; the catalyst A and the catalyst B respectively account for 1-20 wt% of the raw materials; before the reaction, hydrogen is filled in the reaction kettle, the pressure is 1-10MPa, the reaction temperature is 200-260 ℃, and the reaction time is more than 5 minutes and less than 240 minutes.
The main products of the catalytic conversion of the solid cellulosic material are polyols, including ethylene glycol, propylene glycol, butylene glycol and glycerol, wherein the selectivity of ethylene glycol exceeds 50% and the conversion rate of the pretreated solid product exceeds 90%.
Lignocellulose can be utilized by all components through pretreatment, the loss rate of the lignocellulose is lower than 20%, the solvent can be recycled, the loss rate is lower than 5%, and the products are lignin and polyhydric alcohol mainly containing ethylene glycol respectively.
The invention has the following advantages:
1. lignocellulose (including but not limited to crop straws, corncobs, rice straws, wheat straws, miscanthus sinensis, palm barks, pine trees, white birch trees and poplar trees) is used as a reaction raw material, so that the method does not compete with people for grains, is rich in reserves, mostly is agricultural waste, has the advantage of renewable raw material resources compared with an ethylene raw material used in the existing industrial synthesis route of ethylene glycol, meets the requirement of sustainable development, and has important significance on waste utilization and farmer income increase.
2. The pretreatment method has the advantages of low solvent boiling point, easy recovery and use, low loss rate, simple separation process, convenient operation, high lignin purity and high cellulose content of cellulose substances.
3. Under the composite catalyst, the conversion efficiency of the cellulose raw material is high, the selectivity of the ethylene glycol is good, the space-time yield is high, and the industrial utilization is easier.
The present invention will be described in detail with reference to specific examples, which are not intended to limit the scope of the present invention.
Detailed Description
Example 1
Pretreatment of lignocellulose:
before the lignocellulose pretreatment, the lignocellulose needs to be washed, dried and crushed, and taking the miscanthus as an example, the specific process is as follows:
washing 10g of Chinese silvergrass with 50g of water at room temperature, drying at 80 ℃, and pulverizing to obtain particles with particle size less than 1 cm.
The pretreatment process comprises the following steps:
adding 10g of pretreated lignocellulose and 50g of solvent into a flask, adding hydrogen peroxide or/and ozone, wherein the ozone pressure is 0.2MPa, and pretreating at 20-100 ℃ for 1 hour.
And (3) a separation process:
and after pretreatment, filtering to obtain a solution and a solid cellulose substance, washing (or drying) the cellulose substance for subsequent catalytic conversion reaction, carrying out rotary evaporation on the filtrate to recover the solvent, and washing the substances at the bottom of the rotary evaporation kettle to obtain a water-soluble sugar-containing substance and water-insoluble lignin.
Catalytic conversion experiments:
adding 2g of cellulose substances, 0.4g of composite catalyst and 50ml of water into a 100ml reaction kettle, introducing hydrogen to replace the three-time gas, charging the hydrogen to 5MPa, heating to 240 ℃, and reacting for 120 min. After the reaction was completed, the temperature was lowered to room temperature. Centrifugally separating the liquid product from the catalyst, and analyzing and detecting the liquid product on a high performance liquid chromatography calcium type ion exchange column. The product yield is calculated for ethylene glycol and propylene glycol.
Example 2
The compositions of the cellulose substances obtained from miscanthus under different pretreatment conditions were the same as in example 1 (Table I).
TABLE-composition of the cellulose substances obtained from the miscanthus under different conditions of pretreatment (analysis by Van Soest)
As shown in the table I, under different pretreatment conditions, the compositions of the obtained cellulose substances are different, and the cellulose content of 91.5 percent can be obtained by treating the cellulose substances for 2 hours at 80 ℃ in 0.1 percent hydrogen peroxide by taking formic acid as a solvent. Acetic acid, propionic acid, oxalic acid have a slightly lower lignin removal capacity than formic acid, but lignin removal can also be achieved.
Example 3
Composition analysis after pretreatment of various cellulose-based raw materials (Table II), pretreatment conditions were the same as in example 1.
TABLE 2 compositional analysis of pretreated lignocellulosic materials (pretreatment conditions: formic acid, 0.1% hydrogen peroxide, 80 ℃ C., 2h)
As shown in Table II, the pre-treated ligno-cellulosic raw materials have different component contents, wherein the cellulose content of the birch is the highest and reaches 93.7%.
Example 4
The results (Table III) of the catalytic conversion of miscanthus on different composite catalysts to dihydric alcohol were obtained under the same reaction conditions as in example 1, formic acid, 0.1% hydrogen peroxide solution under pretreatment conditions, 80 ℃ and 2 hours.
TABLE three results of catalytic conversion of Miscanthus sinensis to glycols on different composite catalysts (1: 1 mass ratio of catalyst A to B, 0.4g catalyst addition, 50ml water, 10 wt% substrate concentration in water)
As shown in the third table, after pretreatment, the miscanthus sinensis can be efficiently converted into the ethylene glycol, the yield of the ethylene glycol reaches about 60%, and a reaction result similar to that of the reaction when the cellulose is used as a raw material is obtained.
Example 5
The catalytic reaction results of lignocellulose treated under different conditions on Raney Ni + tungstic acid catalyst (Table IV) are the same as example 1, and the ratio of tungstic acid to Raney Ni is 1: 1.
TABLE four catalytic conversion results of lignocellulose treated under different conditions (Raney Ni + tungstic acid as catalyst, ratio of 1:1)
As shown in table four, the pretreatment conditions had a great impact on the subsequent processes, and the formic acid and hydrogen peroxide pretreated lignocellulose could be efficiently converted into ethylene glycol with an ethylene glycol yield of approximately 60%.
Example 6
Comparison of the yields of ethylene glycol obtained by catalytic conversion of cellulosic material under different pretreatment conditions (Table V) was carried out under the same reaction conditions as in example 1.
TABLE five comparison of yields of ethylene glycol from catalytic conversion of cellulosic material under different pretreatment conditions
As shown in Table V, the pretreatment method provided by the invention can remarkably improve the yield of the ethylene glycol, and the yield of the ethylene glycol reaches 65% under similar catalysts and raw materials.
The pretreatment method has the advantages of low boiling point of the adopted solvent, easy recovery and use, low loss rate, simple separation process, convenient operation, high purity of the obtained lignin and high cellulose content of the cellulose substance. Especially, the conversion efficiency of the cellulose raw material is high, the selectivity of the ethylene glycol is good, the space-time yield is high, and the industrial utilization is easier.